Inherently strange crystalline materials called 3D topological insulators (TIs) are all the rage in materials science. This new phase of condensed matter is an insulator in the bulk, yet behaves like a metal on its surface, even at room temperature. The electrons that flow swiftly across the surfaces of TIs are “spin polarized”, meaning the electron’s spin is locked to its momentum, perpendicular to the direction of travel. These electronic states already promise many uses, but ALS researchers working at Beamline 4.0.3 with a team from Berkeley Lab and the University of California, Berkeley have just made an unexpected discovery about TIs that will broaden their possible range of applications: when hit with a photon beam, the spin polarization of the electrons they emit (in a process called photoemission) can be completely controlled in three dimensions, simply by tuning the polarization of the incident light. This strong effect was not what had been assumed about photoemission from topological insulators, or any other material. Controlling the interaction of polarized light and photoelectron spin opens a wide range of possibilities for TIs.

Strengthening Spintronics

The ability to shine polarized light on a topological insulator (TI) and excite spin-polarization-tailored electrons has great potential for the field of spintronics – electronics that exploit spin as well as charge. Devices that optically control electron distribution and flow would constitute a significant advance, allowing more versatile and efficient high-tech gadgets to be created.

Optical control of TI photoemission has more immediate practical possibilities as well. Bismuth selenide could provide just the right kind of photocathode source for experimental techniques that require electron beams whose spin polarization can be exquisitely and conveniently controlled in three dimensions.

The interior bulk of a topological insulator is an insulator, but electrons (grey spheres) move swiftly on the surface as if through a metal. They are spin polarized, however, with their momenta (directional ribbons) and spins (arrows) locked together. Berkeley Lab researchers have discovered that the spin polarization of photoelectrons (arrowed sphere at upper right) emitted when the material is struck with high-energy photons (blue-green waves from left) is completely determined by the polarization of this incident light. (Image Chris Jozwiak, Zina Deretsky, and Berkeley Lab Creative Services Office)

In diagrams of momentum space, TI’s surface electronic states look similar to those for graphene, a single sheet of carbon atoms and another hot topic in materials science. In energy-momentum diagrams of graphene and TI surface states, the conduction bands (where energetic electrons move freely) and valence bands (where lower-energy electrons are confined to atoms) don’t overlap as they do in metals, nor is there an energy gap between the bands, as in insulators and semiconductors. Instead the “bands” appear as cones that meet at a point, called the Dirac point, across which energy varies continuously. Similar as their Dirac-cone diagrams may appear, the electronic states on the surface of TIs and in graphene are fundamentally different: those in graphene are not spin polarized, while those of TIs are completely spin polarized, and in a peculiar way.

Angle-resolved photoemission spectroscopy (ARPES) is used to directly map these states. When energetic photons from a synchrotron light source or laser strike a material, it emits electrons whose own energy and momentum are determined by the material’s distribution of electronic states. Steered by the spectrometer onto a detector, these photoelectrons provide a picture of the momentum-space diagram of the material’s electronic structure.

This diagram shows the electronic states of bismuth selenide in momentum space. ARPES, at left, can directly create such maps with photoelectrons. A slice through the conduction cone at the Fermi energy maps the topological insulator’s surface as a circle (upper left); here electron spins and momenta are locked together. Initial ARPES measurements in this experiment were made with p-polarized incident light in the regions indicated by the green circle and line, where the spin polarization of the photoelectrons is consistent with the intrinsic spin polarization of the surface.

A horizontal slice through the Dirac-cone diagram produces a circular contour. In TIs, spin orientation changes continuously around the circle, from up to down and back again, and the locked-in spin of surface electrons is determined by where they lie on the circle. Scientists call this relation of momentum and spin the “helical spin texture” of a TI’s surface electrons. (Electron spin isn’t like that of a spinning top, however; it’s a quantum number representing an intrinsic amount of angular momentum.)

Directly measuring the electrons’ spin as well as their energy and momentum required an addition to ARPES instrumentation. An efficient, precision detector was developed to measure the spin of low-energy photoelectrons based on how they scatter from a magnetic surface. Called a spin time-of-flight analyzer, this new instrument was first used at the ALS to study the well-known topological insulator bismuth selenide.

While the spin-ARPES experimental results confirmed that bismuth selenide’s helical spin texture persists even at room temperature, they also showed significant deviations between the spin polarizations of the surface electrons versus the photoelectrons. This contrasted standard assumptions in ARPES experiments that the spin polarizations of detected photoelectrons are equivalent to those of electrons within the material, and demanded further study.

Probing the TI surface electrons didn’t require the high photon energy of a synchrotron beam, so researchers moved into a laboratory with a laser that could produce intense ultraviolet light capable of stimulating photoemission, and whose polarization was readily manipulated.

Photoelectron spin flipping mapped through momentum space. Incident light that’s p-polarized produces photoelectrons consistent with the usual picture of spin polarization in TIs surface, but changing the polarization of the incident light also changes the spin polarization of the photoelectrons. (a) Schematic of surface-state helical Dirac fermions in Bi2Se3. (b) Spin-integrated ARPES intensity map of Bi2Se3, taken with laser, s-polarized. Dashed lines are linear guides to the eye illustrating the Dirac cone dispersion of the surface state. (c,d) Corresponding spin polarization (Py) maps taken with p- and s-polarized light, respectively. Dashed guides to the eye are identical to b.

In the first experiments, the incident light was p‑polarized, which means the electric part of the light wave was parallel to a plane that was perpendicular to the TI surface and oriented according to the path of the emitted photoelectrons. Studies of topological insulators often use p‑polarized light in this geometry, so it was no surprise when spin-ARPES measurements showed the photoelectrons were indeed spin polarized in directions consistent with the expected spin texture of the surface electrons.

The beam was then switched to s‑polarized, meaning the electric part of the light wave is perpendicular to the same imaginary plane, and the photoelectrons became spin polarized in the reverse direction—the opposite of what researchers expected. Repeated careful experiments with a range of laser polarizations showed that the spin polarization of the photons in the laser beam did indeed control the polarization of the emitted photoelectrons. When the laser polarization was smoothly varied—and even when it was circularly polarized right or left—the photoelectron spin polarization followed suit.

Probably because the most common kind of spin-ARPES experiment makes a few measurements in a typical geometry using p-polarized light, no results had previously been reported to counter expected surface textures. But with other arrangements, photoelectron spin polarization departs markedly from expectations.

Theory collaborators helped explain the unusual theoretical results when they predicted that just such differences between photoelectron and intrinsic textures should occur. There are also suggestions that the simple picture of spin texture in topological insulators is more complex than has been assumed.